The results from this study indicates that the disposition of 6-MP following I.T. administration was biexponential with a very distinct distributive phase which lasted over 4 hours. The drug levels from both the plasma and CSF declined in parallel. The concentration of 6-MP in the CSF to systemic circulation through the ventricles. subarachnoid space and the arachnoidal villi is slow and probably much smaller in magnitude than the overall diffusional rate of drug transfer from systemic circulation to the CSF.